“Ah, good to know.”
You are talking to your coworker about your weekend plans to drive to the beach. She tells you that there's going to be major construction on some roads that could cause traffic for you. You tell her that you appreciate the information.
Ah, good to know.
“Ah” is a sound English speakers make when they react to interesting new information, or information that they have been looking for.
Ah, okay, he says it’s ten dollars.
If you’re stuck in traffic, and your friend is looking at the traffic app on her phone, she might say:
Ah, there’s an accident two miles ahead.
We also say “ah” to note mild surprise:
Ah, they have my favorite chocolate!
If someone gives you useful information, you can say this. As a full sentence, you would say “That’s good to know.” However, in spoken English “good to know” is more common.
A: She’s really strict about citations so make sure you do those properly.
B: Oh, good to know.
A: If you put out some bowls of vinegar it absorbs the paint smell.
B: Ooh, good to know, thanks!
Usually people say something before “good to know,” such as “ah,” “oh,” “mm,” or “ok.” These noises just show that you listened to what the other person said.